A Mirror Match refers to a match in a game, in which two players face off against each other using the same character (aka dittos), team, etc.
Mirror matches are often done for fun; in high-level play, it also provides the opportunity for players to discover who has the better handle on a given character, and further understand their own playstyles and that of others.
While mirror matches were not technically possible in the original Street Fighter due to there being only one playable character, the two-player mode would pit Ryu against Ken, who at the time shared the same exact moveset, and played exactly the same way.
With Street Fighter II': Champion Edition came the true advent of mirror matches, making it possible for players to challenge each other with the same character. In the event that a mirror match is set up and two players pick the same color character, the second player's color palette changes automatically to prevent confusion. Before Champion Edition made it official, the Super NES version of Street Fighter II: The World Warrior came with a secret code which allowed for Mirror Matches on Versus Mode.
Later games of the Street Fighter series added more potential variables to mirror matches, such as the ISMs of Street Fighter Alpha 3, the different Super Arts of the Street Fighter III series, and the Ultra Combos of Super Street Fighter IV. Many games also have more color palettes to choose from, and characters may have unique win quotes when they defeat their "doubles" in certain modes (e.g: "I am no longer plagued by doubt. Confidence will lead me forward." -Ryu).
In battle, Mirror Matches against good players tend to test the overall knowledge both players have of said character. It is recommended players be familiar with range and Frame Data going into such a match.
Edition Select Edit
Edition Select is a feature in some games such as the Street Fighter 15th Anniversary Collection and Ultra Street Fighter IV that allows the player to select an incarnation of a character. In the 15h Anniversary Collection players are allowed to select a character an an incarnation, from World Warrior to Super Turbo. Properties and frame data of attacks can vary within these characters, such as the visuals of the World Warrior Zangief having the more primitive animation used for said game for his Spinning Piledriver, as well as the distance set after the attack, while the Champion Edition Zangief has the more updated animation and much further distance set if he connects with his piledriver.
For Ultra, characters that were in the regular IV can be selected in that incarnation, characters, such as Ibuki, who debuted in Super instead can be selected from that version upwards. For this version it should be noted that only the incarnation of the most recent version of that game is selectable (for example Ibuki's neckbreaker will have the nerfed Stun damage it recieved during the duration of that edition's supported lifespan). It is recommended that players be familiar with the update history of their character in each game to maximize the best usage of each edition of the character for games that have the feature.